This invention relates generally to thermal transfer ink media which are used to form images by application of thermal energy and, in particular, to a thermal transfer ink medium suitable for transferring printed images onto rough or uneven surfaces and to a method of thermal printing wherein an ink transfer layer and ink are transferred to a transfer medium.
A variety of compact, low cost thermal transfer printing apparatus that use thermal transfer ink media to form images on transfer media by application of thermal energy are available. As shown in FIG. 15, the thermal transfer ink media used in these apparatus generally include a heat-resistant support layer 61 having a thermoplastic ink layer 62 disposed thereon. Thermoplastic ink layer 62 includes at least a thermoplastic material and a coloring material. Thermal energy is applied to support layer 61 from the side opposite ink layer 62 causing a portion of ink layer 62 to melt and transfer to a transfer medium such as paper.
The coloring materials used in thermoplastic ink layer 62 can include pigments, dyes and the like. Some thermoplastic ink layers exhibit adhesive properties when melted and thermal transfer ink media having this type of ink layer are referred to as "thermal melt transfer ink media". Alternatively, the thermoplastic ink layers can include sublimatable dyes which form images by chemical attachment to a transfer media and ink media with this type of ink layer are referred to as "thermal sublimation transfer ink media".
Prior art thermal transfer printing methods generally are not suitable for providing round printed dots or exhibiting good print quality when rough paper, paper with low chemical affinity for the ink or a film is used as the transfer medium. This is due to the fact that the ink from the thermal transfer ink medium is only transferred to the transfer medium at portions where the ink layer of the ink medium is in contact with the transfer medium. No ink is transferred at valley portions of the transfer medium where the ink layer is not in contact with the transfer medium, even if sufficient thermal energy has been applied to the ink layer at that portion. This phenomenon is shown in FIG. 16 wherein an ink layer 74 of a thermal transfer ink medium 72 is not transferred to a transfer medium 73 at a valley portion 75 of transfer medium 73. This occurs even though ink layer 74 has been heated and melted by thermal energy from a thermal head 71 in the portion opposite valley portion 75. The amount of ink transferred is especially small when high resolution dots are transferred.
Sublimation transfer printing is slightly different in that the transfer paper has a developing layer with a chemical affinity for the sublimation colorant. Sublimation transfer paper can also be the cause of difficulties if it has a rough surface since insufficient dye will be transferrd for the color to develop properly. The problems associated with effective ink transfer increase in proportion to the desired transfer resolution.
When the transfer paper is formed of rough fibers, ink that is in contact with the transfer paper transfers as a result of surface adhesion even in non-printing portions. This causes the transfer medium to smear and print stain to develop.
As shown in FIGS. 17A and 17B, three forces act on the ink during transfer from a thermal transfer ink medium 80 to a transfer medium 83 having a plurality of rough spots or points 84. A thermal transfer ink media 80 includes a support layer 81 with a thermal transfer ink layer 82. FA is the adhesive force exerted on ink layer 82 by support layer 81 of thermal transfer ink medium 80. FB is the cohesive force acting within ink layer 82 and FC is the adhesive force between ink layer 82 and transfer medium 83 at each contact point 84. Image transfer can be accomplished when: EQU FC&gt;FA+FB
FA and FC are proportional to the area of the image to be transferred and can be expressed by the equations: EQU FA=fA.times.S; and EQU FC=fC.times.S
wherein fA and fC are the adhesive transfer forces corresponding to FA and FC, respectively, per unit area and S is the area of the image. FB is proportional to the circle of the image when the thickness of the ink is uniform and is expressed by the equation: EQU FB=fB.times..lambda.
wherein fB is the cohesive force of the ink per unit area and .lambda. is the length of the circle.
When an image is formed by circular dots and the image diameter is .phi., the forces acting on the transferred ink can be expressed by the following equations: EQU FA=K.sub.1 .times.fA.times..phi..sup.2 ; EQU FB=K.sub.2 .times.fB.times..phi.; and EQU FC'K.sub.3 .times.fC.times..phi..sup.2
wherein K.sub.1, K.sub.2 and K.sub.3 are constant. The inequality that must be satisfied in order to transfer an image can be expressed as; EQU K.sub.3 .times.fC.times..phi..sup.2 &gt;K.sub.1 .times.fA.times..phi..sup.2 +K.sub.2 .times.fB.times..phi.,
which is equivalent to: EQU K.sub.3 .times.fC&gt;K.sub.1 .times.fA+K.sub.2 .times.fB/.phi.
The value of fC decreases when the transfer medium has a rough surface. Furthermore, .phi. decreases and 1/.phi. increases when the transfer image area is small. Thus, transfer efficiency is especially poor when the transfer medium has a rough surface and the transfer image area is small.
Accordingly, it is desirable to provide an improved thermal transfer ink medium that overcomes the deficiencies of prior art thermal transfer ink media when small ink dots are transferred onto rough surfaces.